Field of the Invention
The invention relates to a device for the automatic relative adjustment of a sample that is positioned on a sample table in relation to an ellipsometer that is analyzing the sample's surface, where the sample position detection system is adjustable in relation to the ellipsometer and is connected to an adjusting device that is assigned to the ellipsometer. The invention also relates to a method for relative adjustment of the sample in relation to the ellipsometer. In addition, the invention relates to a device for detecting the position of a sample to be analyzed relative to a detection system.
Ellipsometry is a sensitive optical method to determine the refractive index and the thickness of very thin film. It utilizes the changes in the polarization state of light after reflection on the sample surface. Collimated and completely polarized light is directed to the sample at a certain angle of incidence. After the reflection, the polarization state of the light beam changes as a function of the sample properties. For example, a linearly polarized incident light is reflected elliptically polarized after interaction with the sample. The polarization changes are detected using suitable configurations of optical polarization components in the path of the beam of the ellipsometer. It is usually described using ellipsometric parameters that in turn allow the calculation of sample properties such as thickness and the refractive index of the films with the assistance of mathematical algorithms.
An often-used configuration of photometric ellipsometers consists of a source for collimated light, a polarizer, the sample, an analyzer and a detector. A periodic signal from which the ellipsometric parameters are derived results at the detector by rotating either the analyzer or the polarizer. An extensive description of ellipsometry can be found in R. M. Azzam, Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam, 1988.
The reference system of ellipsometry is the plane of incidence (x-z plane in the selected coordinate system of the ellipsometer) of the beam. It is created by the axis of the incident collimated light and the plumb on the area element of the sample that the light beam strikes. The angle between the plumb of the axis of the incident beam is called the angle of incidence. All optical polarization components of the ellipsometer are aligned with this reference system. The angle of incidence and the angle position of the components of the ellipsometer in relation to the plane of incidence enter directly into the calculations of the sample properties. These parameters must be known accurately (better than 0.01.degree.) to actually attain the intrinsic accuracy of the instrument. They are either predefined by the design or are entered using a goniometer, where the optical components are arranged. Often, these measures are not adequate making subsequent calibration measurements with known samples necessary.
With ellipsometry, it is possible to measure refractive indexes to an accuracy of 0.01% and coating thicknesses with accuracy in the sub-nm range. A crucial prerequisite for actually reaching the accuracy theoretically possible with ellipsometry is a correct adjustment of the instrument with regard to the sample. Erroneous adjustments can occur, for example, due to operational errors or due to long-term drifts of the mechanical instrument structure. However, small erroneous adjustments are not easily detectable. Thus, the inherent risk exists that they are transferred to the measurement results as systematic errors.
For practical applications, particularly in continuous quality control for the manufacture of thin film, it is crucial to ensure a perfect adjustment of the ellipsometer with regard to the sample even with frequent changes of the sample and over long periods. It is only in this manner that ellipsometry can fully utilize its potential even at high sample throughput.
According to the current state-of-the-art, the assurance of a correct adjustment is only possible using extensive calibration measurements with completely characterized samples. This can only be accomplished with random sampling by an operator.
One solution is the use of a position-sensitive detector in the analyzer arm of a photometric ellipsometer such as was suggested in EP 0 632 256 A1 and U.S. Pat. No. 5,502,567. The circular symmetry of the detector array present in this device in combination with inserted micro-optics supplies a signal that reacts very sensitively to both a tilting of and the distance from the sample. A transparent cone that is directly mounted to a circular array of an even number of identical photo detectors (cone polarimeter) has proven particularly effective. A strictly symmetrical sinus-shaped signal is obtained with correct adjustment of the sample/ellipsometer system, such as the one obtained with a photometric ellipsometer with a rotating polarizer. Thus, the cone polarimeter constitutes an intrinsic reference system of the ellipsometer. A disadvantage is, however, that the responses of the cone polarimeter to changes in the position of the sample are very complex and not linear. The effects of tilting and linear shifts on the signal are not independent of one another, such that the three degrees of freedom cannot be separated out of one single signal. This is particularly true for very strong deviations from the ideal position. Because of this, it is difficult to achieve a control system for automation. The cone polarimeter points to the goal but does not lead the way there.
An additional disadvantage is that with slight misadjustments of the measurement system, whether because of a thermal drift of the ellipsometer/sample system, mechanical influences or even due to an electronic drift, the running measurement must be interrupted to adjust the system using calibration measurements. This is time-consuming and requires qualified operating personnel.
A device, where the readjustment procedure is partially automated is described in the disclosure DE 24 30 521 A1. This device is an arrangement for ellipsometric measurements that is equipped with an automatic adjusting device. The adjusting device operates according to the premise that with a defined spatial incidence angle (defined by an aperture in the beam path of the ellipsometer) the reflected beam to analyzed must be at a defined spatial angle (captured by the detector). With a fixed measurement geometry, the angles which are orthogonal to one another, and which are between the sample surface and plane of the beam as well as between sample surface and the beam itself, are adjusted.
The device to determine the beam deviation consists of four light-sensitive elements, including fiber-optic cables, that are each located in one quadrant, and of a central aperture that defines the emergent spatial angle. With this method, elements located opposite of each other serve the purpose of detecting a deviation of the angle and, by the fact that each pair is connected to a control motor, also the automatic correction of the deviation of the angle. The sample orientation will be changed as long as the intensity distribution at opposing quadrants is not symmetrical.
Thus, this device is only capable of detecting a tilting of the sample, but not a deviation in the height of the sample position. The fact that with this device the height adjustment can only be carried out by hand implies a higher measurement uncertainty and a disproportionately high expenditure if a larger number of samples is to be measured. Furthermore, the detection system and the ellipsometer are not decoupled but have a common beam path. Thus, the alignment of the ellipsometer and detection system to one another is fixed. By coupling both systems, the absolute deviations of the sample positions are always measured from the source of the coordinate system. This leads to fixed positions of the ellipsometer and detection system and an adjustment of the sample can be carried out only via a change in the sample position.
Conventional ellipsometer devices have in common, that the adjustment of the ellipsometer is always carried out via the sample table. For example, if the three reference points on the sample are used, and each reference point has its sensor, then the measurement point must be adjusted according to these three reference points. This works only with flat samples. This system will fail with arched samples. Especially very large samples (large wafers or flat screens) require significant effort. Large wafers with a diameter of 300 mm already have curvatures due to their own weight. Oftentimes, samples are distorted due to mechanical stress after a coating is applied. This would falsify a measurement because during scanning of the sample surface the set sample alignment would not remain. For this reason, the sample tables are equipped with an evacuation device. It vacuum-draws the sample an is pressed flat to the sample table.
This method is expensive and often disadvantageous with sensitive samples. Since such sample tables are not used for transporting the samples during manufacture, the sample must be removed from the production process for measurement, must then be transported to the ellipsometer and returned after the measurement. This often allows only for random sampling. Wafers used in the micro-system technology cannot be measured with such devices because the thin membranes would be destroyed during the evacuation phase.